1. Field of the Invention
The present invention relates to switching circuits for passing or blocking a signal using switching diodes that are turned on or off.
2. Description of the Related Art
FIG. 6 shows the configuration of a known switching circuit. Two conductive patterns 22 and 23 for transmitting signals are formed on an insulating substrate 21. One end of the conductive pattern 22 is provided with a connection land 22a, and one end of the conductive pattern 23 is provided with a connection land 23a. The connection lands 22a and 23a oppose each other. A switching diode 24 is mounted on the insulating substrate 21. An anode terminal 24a is connected to the connection land 22a by soldering, and a cathode terminal 24b is connected to the connection land 23a by soldering.
When the switching diode 24 is turned on by a voltage switching circuit (not shown), a signal input to the conductive pattern 22 is transmitted to the other conductive pattern 23 through the switching diode 24. When the switching diode 24 is turned off, signal transmission is blocked.
In the known switching circuit having the switching diode 24, whose equivalent circuit is shown in FIG. 7, the switching diode 24 has a junction capacitance Cj at a PN junction, an inductance La at the anode terminal 24a, an inductance Lb at the cathode terminal 24b, and a stray capacitance Cs between the terminals 24a and 24b. As shown in FIG. 8, when the switching diode 24 is turned off, the switching diode 24 has the same characteristics as those of a high-pass filter. Thus, the switching diode 24 transmits a high-frequency signal S without greatly attenuating the signal.
Accordingly, it is an object of the present invention to increase the signal blocking effect when a switching diode is turned off.
In order to achieve the foregoing objects, a switching circuit is provided according to the present invention. The switching circuit includes an insulating substrate including two signal transmission lines: a switching diode mounted, in series between the two signal transmission lines, on the insulating substrate, wherein an anode terminal and a cathode terminal are connected to the two signal transmission lines, and the switching diode is turned on or off; and a conductive pattern formed, below the switching diode, on a mounting face of the insulating substrate on which the switching diode is mounted, wherein the conductive pattern is grounded. Accordingly, there are stray capacitances between the conductive pattern and both ends of the anode terminal of the switching diode and stray capacitances between the conductive pattern and both ends of the cathode terminal of the switching diode. These stray capacitances and the inductance at each terminal form a low pass filter. When the switching diode is turned off, the transmission characteristics between the signal transmission lines are such that attenuation can be performed at high frequencies. Accordingly, a high-frequency signal can be sufficiently attenuated.
The conductive pattern may extend to both sides of the switching diode in the direction orthogonal to a direction in which the two terminals of the switching diode are connected with each other, and the conductive pattern may be grounded at both sides of the switching diode. The entire conductive pattern can be sufficiently grounded. As a result, the value of each stray capacitance between the conductive pattern and each terminal can be stabilized.
A ground conductor may be formed on a face opposite to the mounting face of the insulating substrate. Through holes connected to the conductive pattern may be provided at both sides of the switching diode. The conductive pattern may be connected to the ground conductor through the through holes. This eliminates the need to increase the area of the conductive pattern. Thus, a larger area of the mounting face of the insulating substrate can be used for the other circuit wiring.